typing.assert_never()

• python/typing#735 (comment)
  • python/cpython#30842
• https://docs.python.org/3.11/library/typing.html#typing.assert_never

Mypy Exhaustiveness Checking

• Literal: https://mypy.readthedocs.io/en/stable/literal_types.html#exhaustiveness-checking
• Enum: https://mypy.readthedocs.io/en/stable/literal_types.html#id3
• python/mypy#6366

Other languages that use sealed require that all subtypes be defined in the same file. This likely makes it tractable for type checkers to know all subtypes of a sealed class.

Other Languages

Kotlin

• https://kotlinlang.org/docs/sealed-classes.html#location-of-direct-subclasses

Direct subclasses of sealed classes and interfaces must be declared in the same package.

Scala

• https://docs.scala-lang.org/tour/pattern-matching.html

Traits and classes can be marked sealed which means all subtypes must be declared in the same file. This assures that all subtypes are known.

Java 17

• https://openjdk.java.net/jeps/409

A class is sealed by applying the sealed modifier to its declaration. Then, after any extends and implements clauses, the permits clause specifies the classes that are permitted to extend the sealed class. For example, the following declaration of Shape specifies three permitted subclasses:

package com.example.geometry;

public abstract sealed class Shape
    permits Circle, Rectangle, Square { ... }

We should update the rejected ideas section for Union types to include the _T base class method that has been proposed before and highlight the challenges that it has

• https://docs.google.com/presentation/d/1r4fOm4WKjUNBLmsA6oQtOCsu1By0XJLO0KNzQisevgY/edit#slide=id.g142115553c0_1_38

ADT Language Survey

sealed

• Kotlin: https://kotlinlang.org/docs/sealed-classes.html
• Scala 2: https://docs.scala-lang.org/tour/pattern-matching.html
• Java 17: https://openjdk.java.net/jeps/409

enum

• Scala 3: https://docs.scala-lang.org/scala3/reference/enums/adts.html
• Rust: https://doc.rust-lang.org/book/ch06-01-defining-an-enum.html
• Swift: https://docs.swift.org/swift-book/LanguageGuide/Enumerations.html

Pipe (|)

• Haskell: https://wiki.haskell.org/Algebraic_data_type
• OCaml: https://cs3110.github.io/textbook/chapters/data/algebraic_data_types.html
• F#: https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/discriminated-unions

Manual Property

• TypeScript: https://www.typescriptlang.org/docs/handbook/typescript-in-5-minutes-func.html#discriminated-unions

Don't have ADTs

• Go: https://go.dev/doc/faq#variant_types

PEP 622 (which was later superseded) has a complete description for @sealed.

• https://peps.python.org/pep-0622/#sealed-classes-as-algebraic-data-types
• https://github.com/python/peps/blob/main/pep-0622.rst

PEP 591 should be a good example to look at, as @final is similar in scope to @sealed.

• https://peps.python.org/pep-0591/
• https://github.com/python/peps/blob/main/pep-0591.rst